1. Field of the Invention
The present invention relates to a filtering apparatus and a method for a dual-band sensing circuit, more particularly, a gain of a tunable gain amplifier is externally controlled to determine a switch timing for the notch filters for reducing the interference.
2. Description of Related Art
In general, a high frequency communication module will be designed in accordance with the requirement of the operating frequency spectrum. An RF filter is usually used for filtering the required frequencies. If the required filtered frequency spectrum or energy is higher, the operation order of the filters is higher. Moreover, different requirements of the order are applied to different design variants. The loss of signal strength inside the communication module increases with increasing order of the filters. The larger area of the circuit layout will lengthen the signaling path for the RF element, causing a larger loss in signal strength even though the required energy is supplied.
Moreover, when the conventional high-frequency communication module is designed to be co-existed with variable-frequency subsystems, such as the wireless local area network WiMAX, GSM or 3G communication module cooperates with the subsystems in the frequencies 1.8 GHz and 5 GHz, whereby a filter with extra transmission zero is usually employed to prevent interference. However, the added transmission zero will increase the circuitry in the communication module, and thus increase the in-band loss.
While the simultaneous operation of the variant-frequency subsystems will cause interference, this interference won't have too much effect on the whole system. The variable-frequency subsystems don't operate at the same time as above mentioned, but operate alternately. Thus, the general RF filter does not require too much flexibility to handle filtering of the subsystem signals.
In particular, an adaptive notch filter is used for this wireless communication technology. Since the adaptive notch filter is disposed on an RF signaling path, the bandwidth of the filter is simply controlled to filter a specific frequency band and to respond to a required signal, so as to implement an object rather than a common filter.
A common filter operates by either low-pass filtering or high-pass filtering, that is, the filter filters out the higher frequency segment or the lower frequency segment respectively. Nevertheless, the mentioned adaptive notch filter is a special type of filter that can separate the signals into two parts, wherein the lower frequency segment of one part and the higher frequency segment of the other part are filtered out and mixed afterwards.
The technology of the adaptive notch filter used to eliminate the narrowband interference in wideband communication is illustrated in U.S. Pat. No. 6,704,378. The adaptive notch filter selectively filters a received wideband communication signal to eliminate narrowband interference. For determining the presence of narrowband interference, the adaptive notch filter scans various known narrowband channels that lie within the wideband frequency spectrum, thereby finding the interference source by determining the signal strength.
FIG. 1 shows a wireless communication device including an antenna 10 and a low-noise amplifier 12 connected with the antenna 10. Further, the received signals are transmitted to a splitter 14 that is used to split the signals into different signaling paths. Some split signals are transmitted to the adaptive notch filter module 16, and some are transmitted to one narrowband receiver 18. The signals outputted from the adaptive notch filter module 16 are further transmitted to a wideband receiver 19.
The above-mentioned narrowband and wideband indicate two different channels with different frequency spectrums. The signals outputted from every channel are simultaneously transmitted to another system. In this example, the adaptive notch filter 16 can scan every channel to filter the narrowband interference, and couples to another controlling device or other systems such as a network system and telephony system. Moreover, the narrowband receiver 18 can couple with a switch.